There are many fields of technology where it is desirable to generate electrical signals that are proportional to some physical point in a two dimensional planar or non-planar coordinate system. For example, it is often desirable to accurately reconstruct graphs or other technical data representation, to store such data in computers, or to provide touch sensors and the like. A device which has come into use for this purpose is known as an electrographic sensor, wherein orthogonal electrical fields are produced, one in a X direction, and one in a Y direction, in the coordinate system. Contact of the sensor at a specific location with a finger or other object causes the generation of a signal that is representative of the X and Y coordinates of that particular point.
Orthogonal X and Y electrical fields of the devices of this type have been generated by numerous types of systems. For example, parallel electrodes have been placed on opposite edges on two spaced apart sheets. The electrical field in one direction is generated in one sheet with a voltage applied to the set of electrodes on that sheet, and the orthogonal field is generated in the second sheet in a similar manner. In another configuration, the orthogonal electrical fields are generated in a single sheet, with various configurations of electrodes along all of the edges of the sheet, with the potential applied to these electrodes in a proper time sequence. One group of single sheet sensors utilize resistive-type electrodes in contrast to another group that utilize diodes.
In the single sheet type apparatus using resistive electrodes, it is well recognized that equipotential lines generated by the electrodes in the center of the sheet are generally straight parallel lines in each of the directions. However, as the perimeter of the sheet is approached, these equipotential lines become non-parallel curved lines. The curvature, i.e., bow, is produced by the voltage drop along resistor network attached to the resistive electrodes in a direction perpendicular to the applied voltages. If a high linearity device is desired in these prior art devices, such can be achieved near the center of the device, but not at the edge of the device because of these non-parallel bowed equipotentials.
In order to achieve high linearity throughout a larger area of the device, many special systems of electrodes have been devised to increase the region of linearity of the instrument. For example, in U.S. Pat. No. 3,798,370, issued to G. S. Hurst on Mar. 19, 1974, (which patent has a common assignee with this application) electrodes for the application of the voltage to the sheet are arranged in a curve or bow whereby the voltage drops in the resistive element along the edges of the device are at least partially compensated. This is discussed with reference to FIG. 2 therein.
In a like manner, special electrode configurations are shown and described in U.S. Pat. No. 4,079,194, issued to V. Kley on Mar. 14, 1978; and in U. S. Pat. No. 4,178,481, issued to the same inventor on Dec. 11, 1979. In both of thse patents, special electrode configurations are used to reduce the bow to increase the sufficiently linear area of a given sized sensor. Another patent that describes special electrode configurations is U.S. Pat. No. 4,293,734, issued to W. Pepper, Jr., on Oct. 6, 1981. This is one of a series of patents issued to Pepper. These electrodes occupy a considerable space along the edge of the sensor (see FIGS. 3 and 8 of Pepper, for example). Also, in Pepper the network disclosed combines both the peripheral resistance network and the electrodes for introducing potentials into the resistive layer whereby a change in one effects the other and thus does not provide individual adjustment.
Still another patent in the general field of "sensors" is U.S. Pat. No. 4,493,104, issued to L. J. Lukis, et al., on Jan. 8, 1985. In that patent is described a unit having a resistive layer (high resistance) bordered by, and in contact with, a strip of material having a medium resistance. This border is provided with "trimming resistors" to produce (as stated in Col. 3, lines 24 and 30), a linear voltage drop and a linear voltage gradient along each edge. These trimming resistors are formed by overlaying a pair of conductors with a small area of the highly resistive material. According to the patent, the linear voltage drop is then achieved by removing either a portion of the resistive overlay or the conductors themselves. A linear voltage drop, as achieved, actually implies bowed equipotential field lines; this then would require some means for reducing the bow to achieve a maximum active area between the edges of the sensor. The patent requires several depositions of materials; the high resistance layer, the medium resistance border, the conductive regions and the resistive overlay.
Numerous sensors of this general type are utilized for use as an overlay to a video display such as a computer terminal in the form of a cathode ray tube. Such displays usually have outwardly curved (convex) edges. Using sensors of the prior art with inwardly extending electrodes, or at least inwardly directed bowed fields on these display devices, reduces the active area of a sensor to less than the potentially available area.
Accordingly, it is an object of the present invention to produce an electrographic sensor for determining planar coordinates with high linearity, such sensors having a minimum area devoted to means for generating electrical fields therein.
It is another object of the present invention to provide a sensor which may be used over the screen of video display devices which results in an optimum utilization of the space thereof.
It is another object of the present invention to provide a single sheet sensor that can be inexpensively produced which will substantially reduce, or eliminate, the bow of equipotential field lines normally found in prior art devices.
One of the principal objects of the present invention is to provide a touch sensor that will provide the above features and is more readily fabricated.
These and other objects of the present invention will become apparent upon the consideration of the following description with reference to the drawings referred to therein.